S. Kamba, V. Goian, M. Orlita, D. Nuzhnyy, J. H. Lee, D. G. Schlom, K. Rushchanskii, M. Lezaic, T. Birol, C. J. Fennie, P. Gemeiner, B. Dkhil, V. Bovtun, M. Kempa, J. Hlinka, J. Petzelt
Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. The
compressive strain on the film was insufficient to induce ferroelectricity.
Nevertheless, a new polar phonon with frequency near the TO1 soft mode was
detected below 150 K. Two possible explanations of its activation in the
infrared spectra are suggested: splitting of the TO1 phonon due to a lowering
of the crystal symmetry or activation of a phonon from the Brillouin zone edge
as a consequence of an antiferrodistortive phase transition. In the
antiferromagnetic phase we have observed a remarkable softening of the
lowest-frequency polar phonon under an applied magnetic field, which
qualitatively agrees with first principles calculations. This demonstrates the
strong spin-phonon coupling in EuTiO3, which is responsible for the pronounced
dependence of its static permittivity on magnetic field in the
antiferromagnetic phase.
View original:
http://arxiv.org/abs/1201.5458
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